Basanite is an aphanitic (fine-grained) igneous rock that is low in silica and enriched in alkali metals. Of its total content of quartz, feldspar, and feldspathoid (
QAPF), between 10% and 60% by volume is feldspathoid and over 90% of the feldspar is plagioclase. Quartz is never present. This places basanite in the basanite/
tephrite field of the QAPF diagram. Basanite is further distinguished from tephrite by having a
normative olivine content greater than 10%. While the
IUGS recommends classification by mineral content whenever possible, volcanic rock can be glassy or so fine-grained that this is impractical, and then the rock is classified chemically using the
TAS classification. Basanite then falls into the U1 (basanite-tephrite) field of the TAS diagram. Basanite is again distinguished from tephrite by its normative olivine content and from
nephelinite by a normative
albite content of over 5% and a normative
nepheline content under 20%. The
mineral assembly in basanite is usually abundant
feldspathoids (
nepheline or
leucite),
plagioclase, and
augite, together with
olivine and lesser iron-titanium oxides such as
ilmenite and
magnetite-
ulvospinel; minor alkali
feldspar may be present.
Clinopyroxene (
augite) and
olivine are common as
phenocrysts and in the
matrix. The augite contains significantly greater
titanium,
aluminium and
sodium than that in typical
tholeiitic basalt.
Quartz is absent, as are
orthopyroxene and
pigeonite. Chemically, basanites are
mafic. They are low in
silica (42 to 45% SiO2) and high in
alkalis (3 to 5.5% Na2O and K2O) compared to basalt, which typically contains more SiO2, as evident on the diagram used for TAS classification. Nephelinite is yet richer in Na2O plus K2O compared to SiO2. ==Occurrences==